In order to ensure the accuracy in controlling output torque of internal combustion engines and the quantity of exhaust emissions therefrom, it is essential to control a fuel injection mode such as the quantity of fuel to be sprayed from a fuel injection valve or the injection timing at which the fuel injection valve starts to spray the fuel. Accordingly, there have been proposed techniques for monitoring a change in pressure of the fuel upon spraying thereof from the fuel injection valve to determine an actual fuel injection mode.
For example, the time when the pressure of the fuel begins to drop due to the spraying thereof is monitored to determine an actual injection timing. The amount of drop in pressure of the fuel arising from the spraying thereof may be measured to determine the quantity of fuel sprayed actually from the fuel injection valve. Such actual measurement of the fuel injection mode ensures the desired accuracy in controlling the fuel injection mode based on such a measured value.
A fuel pressure sensor (i.e., a rail pressure sensor) installed directly in a common rail (i.e., an accumulator vessel) to measure the above change in pressure of the fuel has a difficulty in measuring the pressure of the fuel accurately because the change in pressure of fuel arising from the spraying of the fuel is absorbed within the common rail. Accordingly, in the invention of Patent Document 1, the fuel pressure sensor is installed in a joint between the common rail and a high-pressure pipe through which the fuel, is delivered from the common rail to the fuel injection valve to measure the fuel pressure change before it is absorbed within the common rail.    Patent Document 1: Japanese Patent First Publication No. 2000-265892